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Version 22 by hannes on 16. April 2008, 00:32

The javascript import features in Helma 2 try to loosely imitate the *semantics of python's import statement|http://www.effbot.org/zone/import-confusion.htm* in order to allow applications to be written in a truly modular way. The magic of python's import statement is that a script never has to do anything special in order to avoid name clashes with other scripts, because every script lives in its one top level scope by default.

To sum it up:

# You don't have to use any special Javascript syntax to denote a namespace. Namespaces are invisible when you're inside, and defined by the file name from the outside.
# It's hardly possible to produce a name collision, as each script lives in its own scope.
# Since scripts live in their own scope, imported scripts are only visible to the scripts that imported them.

=== Implementation

Helma 2 implements this by using a <a href="http://www.mozilla.org/rhino/scopes.html#sharingscopes">separate top-level scope</a> for each loaded script file, using a shared top-level scope as prototype. A simplistic implementation of most of the behaviour described above in the Rhino shell may look like this:

  // set reference to global object
  var __global__ = __global__ || this;
  function include(script, as) {
    var scope = new Object();
    scope.__parent__ = null;
    scope.__proto__ = __global__;
    // scope is now a top level scope that inherits
    // from the global, shared top level scope
    this[as] = scope;

Of course, Helma 2 also takes care of script reloading and stuff, and the shared module scope is really a per-thread scoe that uses another scope for the really shared stuff.

=== Example

Suppose there is a Javascript file called <code>helma/database.js</code> with the following content:

  function getConnection(...) {
  function Connection(...) {
  function Query(...) {

<dd>Makes the functions defined in helma/database.js available in the current scope as
<dt><b>importModuleAs("helma.database", "db")</b></dt>
<dd>Makes the functions defined in helma/database.js available in the current scope as
<dt><b>importFromModule("helma.database", "getConnection", "Query")</b></dt>
<dd>Makes the functions defined in helma/database.js available in the current scope as

Note that the functions in helma/database.js do not care in which namespace they are loaded. They only see what is defined in their local file, and in the global scope shared by all modules. The scope that loaded the module is absolutely invisible to the loaded module!module! Also note that the getConnection() function in the last example can still access the Connection() constructor although it's not included in the importFromModule(). That's the power of closures!